The objective of this project is to provide a detailed, quantitative description of the three-dimensional, time-varying distribution of cardiac potentials in the, ventricular cavities, in the thickness of the ventricular walls, on the epicardial surface, in a torso-shaped electrolytic tank containing an isolated heart, and on the surface of the tank. In the conclusive experiment, all the above, measurement will be, performed in the same isoclated heart preparation. Health relatedness of the project; At present, the interpretation of electrocardiographic signals (body surface ECGs and maps; epicardial and intracavitary electrograms and maps) is hindered by the fact that we are unable to relate these signals to the corresponding intracardiac electrical events with a sufficient degree of certainty. In effect, it has not been possible so far to measure the three-dimensional cardiac electric field at the actual site where it is generate, i.e., in the thickness of the heart walls. This knowledge is indispensable to interpret all the electrocardiographic signals mentioned above in therm of intracardiac events. Experimental design: The experiments will be performed by using in-situ and isolated hearts during spontaneous sinus rhythm and ventricular pacing. Normal hearts and hearts with chronic myocardial infarction or laser-induced injuries will be studied. The intracavitary potential distribution will be measured by means of intraventricular, multi- electrode probes that do not hinder the mechanical activity of the heart. The intramural excitation times and potential distributions will be studied by inserting 50 - 100 closely spaced intramural needles in the ventricular walls and septum. The epicardial distributions will be measured with epicardial socks carrying 200 - 400 electrodes; the extracardiac and "body" surface potentials will be measured using 1200 electrodes regularly distributed in the torso - shaped tank and on its surface. The intramural wavefronts and the related three-dimensional potential and current distributions in the entire intracardiac and extracardiac conducting medium will be displayed using three-dimensional computer graphics and animation. The data will be correlated with histological findings, such as fiber direction and degree of injury in normal and infarcted heart muscle.